Using in situ polarization microscopy and in situ Raman spectroscopy, a polymorphic phase transition from the triclinic to the monoclinic crystal form of 7-alpha-methyl-Delta5,10-norethindrone has been observed in acetone solutions at room temperature. The metastable triclinic form is nucleated epitaxially on the stable monoclinic form beyond a threshold supersaturation. The metastable phase transforms into the stable phase by a solid-solute-solid mechanism in acetone. The thermodynamics of ordinary and epitaxial nucleation are discussed.